Yes you can declassify anything that you classifed, presuming that you are a classifying authority.
I'd like to know what was taken that was classified. If it was just his old scedules and such, so what. But if any of those documents provided information about foreign leaders or foreign activities, or business matters - that's a different kettle of fish. And if it was classified beyond the SECREt level, it's pretty evident that he was not the classifying authority. What I don't get - eveyone I have ever known that had the ability to take home classified (and not TS or TS/SCI) had to have an approved container to keep it in - in other words, a safe to keep hard copies in. But it doesn't appear to me that Biden had any such container in any of the locations. Who knows what else they will find if they search his Rehobeth house. Who has had access to all this stuff?
Joes trying hard to stone wall and act like it isn't any big deal but it raises a lot of questions. I don't think he will ever be punished for it, but it's funny how a lot of Democrats are condeming him on this - perhaps he doesn't have a great deal of support for his next run.